From my experience, I would also like to add, even though it no longer really fits the original topic here:
@der Björn: yes, it is sometimes important how much area the walls take up: we have a plot of 650 sqm, a floor area ratio of 0.2, so you can build over an area of 135 sqm. That is not exactly easy with a bungalow if you are used to spaciousness. I once had a nice Brave-Bungalow, scaled down, calculated by a prefab house manufacturer that sounds Danish but actually comes from the Eastern Bloc. And I really got virtual claustrophobia. Now we are building one and a half stories, which of course is a bit more relaxed.
But now let's come to the wall thickness: this prefab house manufacturer had exterior walls (Energy Saving Ordinance 2009) of 32 cm... + - 0.5 (I don't remember exactly, but around that)... now we are building solid with 17.5 + 14 cm insulation (KfW70)... I don't see a big difference now. Voilà!